Field of the Invention
The present invention relates to a wire electric discharge machine configured to change the relative positions of a wire electrode and a workpiece by a servomotor, while applying a voltage to a machining gap between the wire electrode and the workpiece to generate electric discharge, and machine the workpiece into a desired shape.
Description of the Related Art
In machining power supplies prevalently used in modern wire electric discharge machines, a transistor is controlled to generate desired machining voltage and current. FIG. 3 shows a typical power circuit of a conventional machining power supply.
A machining power supply 1 comprises an electric discharge inducing circuit, configured to induce electric discharge in a machining gap between a workpiece 2 and a wire electrode 3, a current supply circuit for removal machining of the workpiece 2, and the like. The electric discharge inducing circuit comprises a first DC power supply 4 and is connected to the machining gap through a first switching element 5 and a current limiting resistor 6. On the other hand, the current supply circuit comprises a second DC power supply 7 and is connected to the machining gap through a second switching element 8 and a floating inductance 10. In many cases, moreover, the current supply circuit is additionally provided with a third switching element 9 for returning machining current to improve the efficiency of electric discharge machining. Further, the machining power supply 1 comprises a voltage detection circuit (electric discharge detection unit 11) and control unit 12. The electric discharge detection unit 11 detects whether or not electric discharge is generated in the machining gap. The control unit 12 on/off-controls the switching elements 5, 8 and 9.
The following is a description of the operation of the machining power supply shown in FIG. 3. FIG. 4 is a diagram illustrating the operation of the circuit in a case where the third switching element 9 is not used in the machining power supply shown in FIG. 3. FIG. 5 is a diagram illustrating the operation of the circuit in a case where the third switching element 9 is used in the machining power supply shown in FIG. 3. Signals from the control unit 12 for driving the first, second, and third switching elements 5, 8 and 9 are designated by S1, S2 and S3, respectively.
In the case where the third switching element 9 is not used, as shown in FIG. 4, the signal S1 is first output from the control unit 12 to the first switching element 5. If the gap distance of the machining gap is sufficiently small, electric discharge is generated. If the electric discharge in the machining gap is detected by the electric discharge detection unit 11, the control unit 12 is informed of the detection of the electric discharge. The control unit 12 turns off the signal S1 output to the first switching element 5, and at the same time, outputs the signal S2 that is on for a fixed time to the second switching element 8. As the second switching element 8 is turned on, a high current is supplied to the machining gap to perform removal machining of the workpiece. Thereafter, a quiescent time is provided to recover insulation in the machining gap, whereupon one cycle is completed.
In the case where the third switching element 9 is used, as shown in FIG. 5, the signal S3 for the third switching element 9 is turned on at the same time with the signal S2 for the second switching element 8, and the signal S3 is turned off with a delay after the signal S2 is turned off. In this way, a period during which an inter-electrode current is returned can be created, so that the width of the current waveform can be increased to increase the amount of machining. In other words, the efficiency of electric discharge machining can be improved.
In either of the cases where the third switching element 9 is and is not used, high-speed, high-precision machining can be achieved by changing the timing and duration of the signals S1 to S3 for driving the first to third switching elements 5, 8 and 9, depending on the materials, diameters, thicknesses, etc., of the wire electrode and the workpiece or circumstances during machining.
Japanese Patent Application Laid-Open No. 2012-166332 discloses a technique in which a current waveform (wide, low-peak waveform) that facilitates the constituents of a wire electrode to adhere to a core is obtained by controlling the first to third switching elements using the current supply circuit comprising the switching elements. More specifically, the current rise time is reduced, while the reflux time is increased (or the on-times of the signals S2 and S3 in the power supply device shown in FIG. 3 are reduced and increased, respectively). The technique disclosed in Japanese Patent Application Laid-Open No. 2012-166332 is characterized in that a wider current waveform should essentially be achieved by the third switching element. Further, Japanese Patent Application Laid-Open No. 2014-79876 discloses a technique in which a wire electrode is inclined so that its constituents can adhere to both the upper and lower sides of the core.
In the technique disclosed in Japanese Patent Application Laid-Open No. 2012-166332, however, sufficient adhesion to actually hold the core cannot always be achieved. Although the constituents of the wire electrode should be deposited according to welding conditions in FIG. 3 (numerals in the drawings of Japanese Patent Application Laid-Open No. 2012-166332), for example, sufficient constituents are not actually deposited. This is because gradual melting of the wire electrode and the workpiece is essential to the formation of the deposit and reduction of the gradient of the current rise is an effective solution this problem. The necessity of the reduction of the gradient is not mentioned in Japanese Patent Application Laid-Open No. 2014-79876. If the gradient is low (or if the rise is gentle), moreover, it is unnecessary to deliberately provide the third switching element to generate wide pulses.